University of Kentucky
UKnowledge
Agronomy Notes
Plant and Soil Sciences
3-1985
Mineralogy and Soil Productivity
Anastasios D. Karathanasis
University of Kentucky, [email protected]
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Karathanasis, Anastasios D., "Mineralogy and Soil Productivity" (1985). Agronomy Notes. 75.
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UNIVERSITY OF KENTUCKY
COLLEGE OF AGRICULTURE
COOPERATIVE EXTENSION SERVICE
Lexington, Kentucky 40546
AGRONOMY NOTES
Vol. 18
No.2, March 1985
MINERALOGY AND SOIL PRODUCTIVITY
A. D,. Karathanasis
The soil that is often called "dirt" is not as dirty as many people think. If we
remove the 5 to 10% of the soil material, composed of organic matter and amorphous Fe,
Al and Si hydrous oxides, the remaining 90 to 95% is a mixture of crystalline or
subcrystalline minerals. We are able to see and identify with the naked eye, the large
crystalline particles of the sand fraction. For the smaller mineral particles of the
silt and clay fractions microscopic or other more sensitive methods (x=ray diffraction,
thermal) are needed for their identification and quantification. While little
attention is often given to these crystalline minerals as to their relation to soil
productivity, their different morphological and physicochemical properties playa very
important role ,in 60ntrolling soil behavior. Sometimes the presence of even very small
amounts of certain minerals in the soil is more critical than the presence of large
amounts of others because of unique characteristics affecting their reactivity. For
example. the mineral smectite (montmorillonits) with its high cation exchange capacity,
surface area, and shrink-swell potential is a determining factor in ion exchange
reactions, sorption processes and mechanical strength of soils. On the other hand
quartz, the dominant mineral in many soils. is relatively non-reactive and is largely
considered as a dilutant of other more reactive mineral components in soils. Other
soil minerals such as mica and kaolinite are intermediate in reactivity between those
of smectite and quartz.
Generally, the effeot of the soil mineralogical composition on soil productivity
is two-fold: (1) it influences chemical reacytbns regulating nutrient availability and
uptake and (2) it affeots physical properties controlling soil moisture balance and
physical oonditions of the soil.
Soil Mineralogy and Nutrient Ayailability
Two aspects which relate soil mineralogical composition to nutrient availability
are noteworthy. First, soil minerals are the source from which most plant nutrients
are released in available forms into soil solution. Such release comes about through
mineral transformation. weathering, or dissolution - reprecipitation reactions, i.e.
K-release from weathering of micas; P and Ca release from phosphate and carbonate
minerals. Secondly, due to their ion exchange properties and surface reactivity, soil
minerals control the ionic equilibrium of the soil solution. This is the equilibrium
on which mineral uptake necessary for plant growth depends. It is regulated by ion
exchange and fixation processes.
The available nutrient supply in soils is considered to be the sum of both soluble
and exchangeable forms. Roots of living plants absorb nutrients from the soil's
solution phase which is in equilibrium with the exchange phase, with the plant root
itself being a cation exchanger. The capacity of the soil to replenish the solution
nutrient concentration after it has been lowered by plant nutrient uptake depends on
The College of Agflcullure 1$ an EQ(JiJl Opporiumly Orgamzation With respect 10 educallon and employmen randi:; aulhoflzed 10 prOVide research, educelio~al mformalion and other ~~rvi~es only to
,nd'vlduAiS and msll/utlons Ihall/mc/JOn WIIMvl regard lor3e8, COIOf, natlona/oflgm, sex, religion, age and handicap Inqwriesregarding compliance WJ!h Tille V/ and Tille VII ollhe CIVIIRlgMsAct of
1964, Tille IX ot/he Educat,onal Amendments, Sec/lon 504 01 IheRehabilllallon Ad and olhet relaled malletS should be directed 10 Equal Opporlumly Of/Ice, College of Agriculture, Umvelslty of
Ken/Ucky, Aoom 5-105, Agncullural SCience Blllldmg-Notth, Lexmglon, KenllJcky 40546
UNIVERSITY OF KENTUCKY, KENTUCKY STATE UNIVERSITY, U S DEPARTMENT OF AGRICULTURE, AND KENTUCKY COUNTIES. COOPERATING
"\
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the mineralogioal oomposition of the soil. This is beoause a great portion of the
exohange oapaoity of the soil whioh determines the ability of the soil to supply
growing plants with nutrients is related to orystalline and suborystalline mineral
oomponents. The magnitude of the ion exohange reaotion is determined by the type of
ions involved, nature of the soil solution and the strength of attraotio~ between a
partioular mineral and a speoifio oation. Generally minerals with high oatio~+exoh2~ge
oapaoity (CEC) suoh as smeotite, have a greater attraotion for polyvalent (Ca ,Mg )
than monovalent (K+, Na+) ions. The reverse is true for minerals with low CEC suoh as
kaolinite.
Equally important is the role of soil minerals in oontrolling anion exohange and
fixation phenomena. Minerals with low CEC and low orytallinity often have higher
affinity for anions (e.g., H P04-), especially under aoid pH oonditions • .The greater
2
the negative charge per unit of surfaoe area of the mineral (e.g., smeotite) the more
anions are excluded from the partiole surface and lost by leaching from the rooting
zone.
Mineralogy and Physical Soil Properties
Most physioal soil properties are determined largely by themineralogioal
oomposition of the soil. Soil texture, structure, moisture charaoteristics, pore size
distribution, plastioity, shrink-swell potential, soil strength', and erodibilty can be
direotly or indirectly related to· soil mineral properties. Particle size distribution
results from a physioal degradation of soil minerals due to imperfect lattice energy
distributions in the silicate layers causing strains in the mineral structure. Soil
strength, aggregation and plasticity are affected by the various attraotive and
repulsive foroes exerted by the·various minerals. The overall struoture of the soil is
the· produot of the arrangement and bonding of individual soil partioles into aggregates
caused by ionic, organic, water and clay mineral surfaoe interactions. The individual
shape and stability of soil aggregates, however, varies with soil mineralogioal
composition. Smectitio minerals induce formation of spherioal aggregates, whereas
kaolinite promotes a platy struoture. Structural stability of soil aggregates is a
function of surface charge (CEC) of the minerals oontained. It decreases in the order
montmorillonitio > illitio > kaolinitio soils.
Soil water retention oharacteristios are a!so affeoted by the olay mineralogioal
make-up of the soils. This is due to variations in speoific surfaoe area and
solute-water-solid interaotions of speoifio olay minerals. Smeotite soils retain muoh
more water than illitio or kaolinitic soils, as is illustrated in Table 1. Beoause of
this, water adsorption charaoteristios can be used for help in identifying soil
mineralogioal oomposition.
Shrink-swell charaoteristios of a soil is another important physical property
directly related to soil mineral content. It is due to charge imbalances in the
crystal lattioe and surfaoe oharge density of olay minerals. Volume expansion and
shrink-swell potential are muoh higher in smeotitio than ,in illitio or kaolinitio soils
beoause of their higher surfaoe ohange density (Table 1). Soil erodibility is also
affeoted by mineralogioal composition, because the mineralogy of the soil influenoes
such properties as aggregate stability, permeability and soil shear strength •. Smeotite
soils, despite their higher aggregate stability, are more susoeptible to surface 5011
losses beoause of their slow permeability. Total erosional losses however, are
generally greater from illitic or kaolinitio soils beoause they require less water to
reach saturation. At that point their shear strength is reduoed to near zero, and they
erode with surfaoe water flow (Table 1).
3
Mineralogy of Kentucky Soils
Very few mineralogical data have been published for soils in Kentucky. As an
initial attempt to classify mineralogy of the subsoil, a general mineralogy map of the'
state was constructed based on currently available information (Fig. 1)'. The map
suggests that quartz, mica, and feldspars are the dominant minerals of the sand and
silt size fractions and that illite, smeotite, kaolinite and hydroxyinterlayered
vermioulite or smeotite dominate the olay size soil fraction. Soils of the Western
Coalfields, Eastern Coalfields, and Eastern Pennyrile regions generally oontain more
quartz in the sand and silt fraotion than soils of the Purohase, Western Pennyrile, and
Bluegrass regions. The sand and silt fractions of the latter regions, although still
dominated by quartz, oontain signifioant amounts of potassium feldspars and mioa. The
feldspar component is generally more prominent in soils of Western Kentuoky, With mioa
being more prominent in oentral and eastern parts of the state. Soils With high
feldspar or mioa content are considered to have adequate water-soluble, exchangeable,
and non-exchangeable K-supplying capaoity because of the potassium released from their
mineral structure during weathering. However, muscovite-type micas are more resistant
to weathering than feldspars, with a rate of K-release not fast enough to replenish the
solution K as it is removed by plants. Furthermore, not all mica-type minerals contain
the same amount of K in their crystal structure (muscovite 11%, glauconite 5%, biotite
8%). Potassium availability to plants in these soils also depends on the rate and
duration of ion exohange reaotions, the nature of whioh is affeoted by soil
mineralogioal composition, K-specifioity for certain exohange mineral sites and the
nature of other ions in solution.
To date no definite regional trends have been established for the mineralogioal
oomposition of the clay size fraotion of Kentuoky soils. Generally, however, there is
a tendenoy for both Eastern and Western Coalfield regions to contain less smeotite and
more prominent amounts of hydroxyinterlayered olay minerals than other regions.
Smeotite soils are most often found in some floodplain areas of the Mississippi, Ohio
and Green rivers. Although when aoid, these soils require more time to raise pH because
of their high cation exchange oapaoity, most do not have very low base saturation.
Most of the soils in other regions contain variable quantities of illite,
smectite/vermiculite, kaolinite and hydroxyinterlayered olay minerals. Amounts of lime
needed to neutralize excessive acidity in these soils will be determined by the
original aoidity present and the buffering/capacity of the mineral mixture.
Some soils of the Outer Bluegrass region formed from shaly parent materials are
relatively high in vermioulite and interstratified minerals that exhibit a significant
K-fixation capacity. Higher rates of K-fertilization may be required for such soils.
Soils of the Hills of the Bluegrass, however, contain significant amounts of readily
available potassium and require smaller rates of K-fertilization. Bluegrass soils
developed from phosphatic limestone are likely to maintain relatively high P levels in
the soil solution even without fertilization, depending on the solubility of the
phosphate minerals present. Liming acid soil surfaces improves P availability because
it deoreases phosphate fixation by Al and Fe hydroxyoxides. Unless limed adequately,
however, higher rates of P fertility are required for optimum crop growth.
There is still much to be learned about physico-chemical reactions controlling soil
behavior. Some of these reactions are triggered by various types of physical or
chemical soil management imposed by man. To successfully quantify them and predict how
soils will respond, a quantitative knOWledge of the mineralogical composition of the
soil system will be necessary.
4
Table 1:
Effect of clay mineralogical composition on some physical soil
properties.
Water Content
at different tensions
bars
Dominant Mineralogy
0.1
0.33
15
smectitic
(montmorillonitic)
>50
>35
>18
illitic or mixed
35-50
22-35
9-18
kaolinitio
<22
<15
<9
($)
At Zero
Shear Strength
>20
5-20
<5
Volume
Expansion
>150
70-150
<50
5UOSOIl MIN£RAl MAP OF
KENTUCKY
5
7
DONINANT MJtIERAlOGY
Area
l.
2.
3.
4.
5.
6.
7.
8.
9.
10.
Sand and Silt FI'dction
<GOt quartz; mica
40·80'," quartz; /Ill Cd
40-80k quartz: feldspars
40-80:~
40-80~
quartz; feldspars
quartz: mica
40·80::: quarlz;,mica; feldspars
40·801. quartz; mied; feldspars
40-80~'J quartz; mica: feldspars
40-80'. quartz; mi Cd
<60~
quartz: mica
smectite ~ illite:> kaolinite
illite .' kaolinite ~ smectite,. HEX
i 11 He ~ HEX,. kilOl illite
HEX:> illite> kaolinite
kaolinite ~ illite:> smectite:> HEX
HEX:> kaolinite:> illite
ill te" sl'leclite'> kaolinite:> HEX
ill te :> HEX:> kaolinite:> smectite
ill tc :> ka01 initc :> HEX
kilO inite ... smectite:> HEX:> illite
._~-----~~~~~
t
HEX - hydroxyinterlayered lIIinerals
%